1. Field of the Invention
The present invention relates to an ink-jet recording head for performing recording on a recording medium by discharging ink onto the medium from the recording head. The invention also relates to an ink-jet recording apparatus using the recording head. The term "recording" throughout this specification is used in the sense that ink or the like is provided (printing) for all materials which can accept ink thereon (recording media), such as cloth, paper, yarn, sheet materials and the like. The present invention is applicable to these uses.
2. Related Background Art
Among currently known various recording methods, an ink-jet recording method, which is a non-impact recording method producing very little noise during recording, is known to be extremely effective since it is possible to perform high-speed recording without requiring specific ink-fixing on ordinary paper.
FIGS. 1 and 2 schematically illustrate the construction of a typical ink-jet head used in such an ink-jet recording method. FIG. 1 is an outer perspective view, and FIG. 2 is a perspective view showing the interior construction.
The ink-jet head shown in FIGS. 1 and 2 includes a base plate 200 used for constructing respective components (described below) to be formed thereon, and an element substrate (hereinafter referred to as "the heater board") 100 disposed on the base plate 200, the heater board being provided with a plurality of electro-thermal transducers (heaters) 101 used as discharging-energy generating elements. Ink channels 303 corresponding to a plurality of respective ink-discharge openings 301 are formed on the heater board 100. A grooved member (a top plate 300) having a plurality of grooves therein for forming the above-mentioned ink channels is bonded to a predetermined position of the heater board 100. The top plate 300 has an orifice plate 304 having the ink-discharge openings 301 formed therethrough, a recess for forming a common liquid chamber 302 for storing ink to be supplied to the ink channels 303, and a cylindrical ink-filling opening 305 through which ink is supplied into the chamber 302. The above-described bonding of the top plate 300 onto the heater board 100 is carried out by the following process. For bonding the top plate 300 to the heater board 100, the top plate 300 is first allowed to temporarily adhere to the board 100 in such a manner that a plurality of heaters 101 can correspond to the respective ink-discharge openings 301. Then, mechanical pressure is applied to the top plate 300 from above by an urging spring (not shown), thus obtaining a sufficiently intimate connection therebetween. Subsequently, the top plate 300 and the heater board 100 are sealed therearound by a sealing agent 400, as shown in FIG. 7, and whereby the ink channels 303 and the common liquid chamber 302 are hermetically cut off from the exterior.
The generation of the pressure required for discharging ink in the ink-jet recording head results from the fact that thermal energy generated in the heaters 101 acts on the ink flowing in the ink channels 303 to induce film boiling, which further produces bubbles. The thus-generated pressure is transferred in the direction of the ink-discharge openings 301 through the ink flowing in the channels 303 and also in the direction of the common liquid chamber 302, the two directions being opposite to each other.
The ink flowing in the channels 303 is squeezed out from each of the discharge openings 301 by the action of the pressure transferred to the discharge opening 301 so as to form flying discharge droplets. At the time when the ink is formed into a discharge droplet which then departs from the discharge opening 301, the meniscus formed on the surface of the ink at each opening 301 recedes according to the amount of droplet. By the action of the tension for pulling back the meniscus in the direction of the discharge opening 301, the ink is again filled in the ink channel after a lapse of a certain time as it has been before discharging. Such a phenomenon is referred to as "refilling". In the actual recording operation, the above-described process is repeated while good condition of refilling is ensured, thereby achieving continuously stable ink discharging.
In order to cope with recent trends towards an increased amount of discharging ink and higher printing speed, a large amount of ink is discharged for a short time, and accordingly, refilling should be performed at higher speed. However, conventional heads often fail to perform stable refilling in the above background, which brings about unstable ink discharging and further causes a deterioration in printing quality. Further, there arises a disparity between the amount of ink discharged for the first time after recording has been started and the amounts of ink for subsequent numbers of discharging time, which may disturb a resultant recorded image.
It is considered that the above-mentioned drawbacks originate from pressure waves (back waves) transferred in the direction opposite to that of the ink-discharge openings.
Such pressure waves impede the ink from flowing into the ink channels, thus making it difficult to perform refilling at higher speed.
Also, for the same reason, there disadvantageously arises a disparity between the amount of discharging ink for the first time after recording has been started, which discharging operation is free from the effect of the back waves, and the amounts of ink for subsequent number of times, which discharging operations are adversely influenced by the back waves.
One of the measures to effectively reduce the influences of the pressure waves is a small chamber (hereinafter referred to as "a buffer chamber" or "a bubble cell") which is communicated only to the common liquid chamber and contains bubbles (gas) for eliminating the pressure of the back waves, as disclosed in Japanese patent Application Laid-Open No. 1-308644.
Since this small chamber is communicated to the common liquid chamber through a very small communicating portion, it is formed in such a shape that it is very difficult for ink to enter. With this construction, a gas is likely to be constantly present in the small chamber and functions to eliminate pressure fluctuations caused by the back waves produced during the ink discharging, thus obtaining stable refilling and further achieving excellent high-speed printing.
In order to remove bubbles entering the ink channels (flow channels) and also to obviate thickened ink within the flow channels in the vicinity of the discharge openings, a recovering operation by vacuum suction is performed to such and discharge the ink to the exterior from the discharge openings.
However, when this recovering operation is performed on a head provided with the above-described small chamber, a considerable amount of gas is inevitably vented from the chamber, thus disadvantageously reducing the effect of eliminating the pressure of the back waves.
In order to overcome the above drawback, the communicating portion leading to the common liquid chamber may be formed in a more complicated shape so that bubbles can be prevented from being vented from the chamber even by performing this recovering operation. However, this makes it difficult to manufacture such a chamber and also conversely may weaken the effect of eliminating the pressure of the back waves.
Additionally, in order to ensure the stable function of the buffer chamber after the recovering operation, it is necessary to form the buffer chamber with a large volume to such a degree that some bubbles can still remain even after a certain amount of bubbles are vented from the chamber.
In general, it is necessary that the amount of ink (suction amount) required for the above-described ink discharging be larger than the total of the volume of the common liquid chamber including the buffer chamber and that of the ink channels. Thus, the larger the buffer chamber, the larger the amount of discharging ink required. This necessitates a larger volume pump for use in the suction operation and also gives rise to an increase in the amount of ink which cannot be discharged, but instead should be exhausted.
In the ink-jet recording head, if bubbles, such as air, are present in the ink flowing from the common liquid chamber 302 to the ink-discharge opening 301 without performing ink discharging for a long period of time, the bubbles may gradually grow with a lapse of time to disturb the flow of ink and further inhibit the ink from being discharged. In order to avoid such a situation, the ink-jet recording apparatus usually performs a recovering operation for sucking the ink at regular intervals to remove the bubbles.
The same also applies to the air within the buffer chamber. That is, if the buffer chamber is left for a long period of time without performing ink discharging, the air causes bubbles to grow and to reach the ink channels, which may further prevent the ink from being discharged. For this reason, the buffer chamber is located in the farthest-possible position away from the ink-discharge openings 301.
However, heater boards are becoming smaller to be adaptable for smaller-sized ink-jet recording heads and also to decrease the cost. Along with such downsizing of the board, the buffer chamber is required to be placed in the vicinity of the ink-discharge openings 301. This may cause the bubbles which have grown to reach the portion near the ink-discharge openings from the buffer chamber between the recovering operations, thus resulting in a failure in discharging the ink. In addition, since the volume of the common liquid chamber 302 is becoming smaller to be adaptable for the downsizing of the heater board, a small amount of bubbles stored in the common liquid chamber 302 may reach the portion near the ink channel, thus also bringing about a failure in discharging the ink.